Marshal variables

[trojanc]

Motivation/Abstract

please describe in 1-3 sentences,

• Variables are not marshaled and loaded back
• This change adds to add feature to export and import BPMN state, incl. resume capability #12

Examples: This is a bug fix for ... ; I want to use this BPMN extension attributes 'X' for my project 'Y'...

Description/Comments

In this PR I added marshaling for VariableHolder when the engine is marshaled and unmarshaled
A unit test is also provided to confirm if the engine is loaded correctly after being marshaled

Something abnormal that I did:
I moved VariableHolder to the bpmn_engine package. This was to avoid having to put marshaling code in the var_holder/holder.go file. I felt like these files are kept so clean with the all the marshaling code in the mashalling.go file, I wanted to have the var_holder/holder.go file clean of marshaling logic too. I couldn't reference VariableHolder properly for marshaling from the bpmn_engine package otherwise...

A much smaller change would have been to put the marshaling functions in var_holder/holder.go..

Checklist

Depending on your PR, please ensure the overall consistency/integrity of the project remains.
Please tick just one check item per section below

Tests

• did you update or create tests for your code changes?
• not relevant

Code examples

• did you update or add example code snippets, which relate to your code changes
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Documentation

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• not relevant